Answer
(Fatwa: 421/291/L=1431)
The process of halalah is that after passing the iddah period woman will marry a man, and he after having intercourse divorces her with his consent with his consent, in this case she can marry the first husband after observing iddah period. If the second husband penetrates only his head of penis into her vagina, then after divorce having completed her iddah period she can marry the first husband.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband